Haris Aziz TA Mian Wasif Contents 1 Fundamentals of NC Technology 2 Computer Numerical Control 3 DNC 4 Applications of NC 5 Engineering Analysis of NC Positioning Systems ID: 671043
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Slide1
Numerical Control
Instructor: Dr
Haris
Aziz
TA:
Mian
WasifSlide2
Contents
1.
Fundamentals of NC Technology
2.
Computer Numerical Control
3.
DNC
4.
Applications of NC
5.
Engineering Analysis of NC Positioning Systems
6.
NC Part ProgrammingSlide3
Numerical Control (NC) Defined
Programmable automation
in which the mechanical actions of a ‘machine tool’ are controlled by a program containing
coded alphanumeric data
The alphanumeric data represent
relative positions
between a
workhead
(e.g., cutting tool) and a
workpart
When the current job is completed, a new program can be
entered for the next jobSlide4Slide5
Basic Components of an NC System
Machine
Control Unit
Program
Instructions
Processing Equipment
1.
Program of instructions
Part program in machining
2.
Machine control unit
Controls the process
3.
Processing equipment
Performs the processSlide6
NC Coordinate System
For flat and prismatic (block-like) parts:
Milling and drilling operations
Conventional Cartesian coordinate system
Rotational axes about each linear axis
Right Hand Rule
For rotational parts:
Turning operations
Only
x
- and
z
-axesSlide7
Motion Control System
Point-to-Point systems
Also called position systems
System moves to a location and performs an operation at that location (e.g., drilling)
Also applicable in robotics
Continuous path systems
Also called contouring systems in machining
System performs an operation during movement (e.g., milling and turning)Slide8
Interpolation Methods
Linear interpolation
Straight line between two points in space
Circular interpolation
Circular arc defined by starting point, end point, center or radius, and direction
Helical interpolation
Circular plus linear motion
Parabolic and cubic interpolation
Free form curves using higher order equationsSlide9
Absolute vs. Incremental Positioning
Absolute positioning
Move is: x
= 40,
y
= 50
Incremental positioning
Move is: x
= 20,
y = 30.Slide10
Computer Numerical Control (CNC)
Storage of more than one part program
Various forms of program inputProgram editing at the machine toolFixed cycles and programming subroutines
Interpolation
Acceleration and deceleration computations
Communications interface
DiagnosticsSlide11
Machine Control Unit of CNCSlide12
DNC>CNC>DNC
Direct numerical control (DNC) – control of multiple machine tools by a single (mainframe) computer through direct connection and in real time
1960s technology
Two way communication
Distributed numerical control (DNC) – network consisting of central computer connected to machine tool MCUs, which are CNC
Present technology
Two way communicationSlide13
Direct NCSlide14
Distributed NCSlide15
NC Applications
Machine tool applications:
Milling, drilling, turning, boring, grinding
Machining centers, turning centers, mill-turn centers
Punch presses, thermal cutting machines, etc.
Other NC applications:
Component insertion machines in electronics
Drafting machines (x-y plotters)
Coordinate measuring machines
Tape laying machines for polymer composites
Filament winding machines for polymer compositesSlide16
Common NC Machining Operations
Turning
Milling
DrillingSlide17
CNC Horizontal Milling MachineSlide18
NC Application Characteristics (Machining)
Where NC is most appropriate:
1.
Batch production
2.
Repeat orders
3.
Complex part geometries
4.
Much metal needs to be removed from the starting
workpart
5.
Many separate machining operations on the part
6.
The part is expensiveSlide19
Cost-Benefit of NC
Costs
High investment cost
High maintenance effort
Need for skilled programmers
High utilization required
Benefits
Cycle time reduction
Nonproductive time reduction
Greater accuracy and repeatability
Lower scrap rates
Reduced parts inventory and floor space
Operator skill-level reducedSlide20
NC Part Programming
Manual part programming
Manual data input
Computer-assisted part programming
Part programming using CAD/CAMSlide21
Manual Part Programming
Binary Coded Decimal System
Each of the ten digits in decimal system (0-9) is coded with four-digit binary number
The binary numbers are added to give the value
BCD is compatible with 8 bits across tape format, the original storage medium for NC part programs
Eight bits can also be used for letters and symbolsSlide22Slide23
Creating Instructions for NC
Bit - 0 or 1 = absence or presence of hole in the tape
Character - row of bits across the tape
Word - sequence of characters (e.g., y-axis position)
Block - collection of words to form one complete instruction
Part program - sequence of instructions (blocks)Slide24
Block Format
Organization of words within a block in NC part program
Also known as tape format because the original formats were designed for punched tape
Word address format - used on all modern CNC controllers
Uses a letter prefix to identify each type of word
Spaces to separate words within the block
Allows any order of words in a block
Words can be omitted if their values do not change from the previous block Slide25Slide26
Types of Words
N - sequence number prefix
G - preparatory words
Example: G00 = PTP rapid traverse move
X, Y, Z - prefixes for
x
,
y
, and
z
-axes
F - feed rate prefix
S - spindle speed
T - tool selection
M - miscellaneous command
Example: M07 = turn cutting fluid onSlide27
Example: Word Address Format
N001 G00 X07000 Y03000 M03
N002 Y06000Slide28
Cutter Off-Set
Cutter path must be offset from actual part outline by a distance equal to the cutter radiusSlide29
Issues in Manual Part Programming
Adequate for simple jobs, e.g., PTP drilling
Linear interpolation
G01 G94 X050.0 Y086.5 Z100.0 F40 S800
Circular interpolation
G02 G17 X088.0 Y040.0 R028.0 F30
Cutter offset
G42 G01 X100.0 Y040.0 D05Slide30
Computer Assisted Part Programming
Write machine instructions using natural language type statements
Statements translated into machine code of the MCU
APT (Automatically Programmed Tool) Language
The various tasks in computer-assisted part
programming are divided between;
1) The human part programmer
2) The computerSlide31
Sequence of activities in computer-assisted part
programmingSlide32
Part Programmer’s Job
Two main tasks of the programmer:
1.
Define the part geometry
2.
Specify the tool pathSlide33
Defining Part Geometry
Underlying assumption: no matter how complex the part
geometry, it is composed of basic geometric elements and
mathematically defined surfaces
Geometry elements are sometimes defined only for use in
specifying tool path
Examples of part geometry definitions:
P4 = POINT/35,90,0
L1 = LINE/P1,P2
C1 = CIRCLE/CENTER,P8,RADIUS,30Slide34
Specifying Tool Path and Operation Sequence
Tool path consists of a sequence of points or connected
line and arc segments, using previously defined geometry
elements
Point-to-Point command:
GOTO/P0
Continuous path command
GOLFT/L2,TANTO,C1Slide35
Other Functions in Computer Assisted Part Programming
Specifying cutting speeds and feed rates
Designating cutter size (for tool offset calculations)
Specifying tolerances in circular interpolation
Naming the program
Identifying the machine toolSlide36
Computer Task in Computer Assisted Part Programming
1.
Input translation - converts the coded instructions in the
part program into computer-usable form
2.
Arithmetic and cutter offset computations - performs the
mathematical computations to define the part surface and
generate the tool path, including cutter offset
compensation (CLFILE)
3.
Editing - provides readable data on cutter locations and
machine tool operating commands (CLDATA)
4.
Postprocessing
- converts CLDATA into low-level code
that can be interpreted by the MCUSlide37
NC Part Programming Using CAD/CAM
Geometry definition
If the CAD/CAM system was used to define the original
part geometry, no need to recreate that geometry as in
APT
Automatic labeling of geometry elements
If the CAD part data are not available, geometry must
be created, as in APT, but user gets immediate visualfeedback about the created geometrySlide38
Tool Path Generation Using CAD/CAM
Basic approach: enter the commands one by one (similar
to APT)
CAD/CAM system provides immediate graphical
verification of the command
Automatic software modules for common machining
cycles
Profile milling
Pocket milling
Drilling bolt circlesSlide39
NC Part Programming using CAD/CAMSlide40
Example of Machining Cycle in Automated Part Programming Module
Pocket milling
Contour turningSlide41
Example of Machining Cycle in Automated Part Programming Module
Facing and shoulder facing
Threading (external)Slide42
Manual Data Input
Machine operator does part programming at machine
Operator enters program by responding to prompts and questions by system
Monitor with graphics verifies tool path
Usually for relatively simple parts
Ideal for small shop that cannot afford a part programming staff
To minimize changeover time, system should allow programming of next job while current job is runningSlide43
Analysis of NC positioning
Two types of NC positioning systems:
1.
Open-loop - no feedback to verify that the actual
position achieved is the desired position
2.
Closed-loop - uses feedback measurements to
confirm that the final position is the specified position
Precision in NC positioning - three measures:
1. Control resolution
2.
Accuracy
3.
RepeatabilitySlide44
Open loop Motion Control System
Operates without verifying that the actual position
achieved in the move is the desired positionSlide45
Example: open loop positioning
The worktable of a positioning system is driven by a leadserew
whose pitch =6.0 mm. The leadscrew is connected to the output shaft of a stepping motor through a gearbox whose ratio is 5:1 (5 turns of the motor to one turn of the leadscrew). The stepping motor has 48 step angles. The table must move a distance of 250 mm from its present position at a linear velocity = 500 mm/min Determine (a) how many pulses are required to move the table the specified distance and (b) the required motor speed and pulse rate to achieve the desired table velocity.Slide46
(a) the teadscrew rotation angle
A correspondingto a distance x = 250 mm,Slide47
(b) The rotational speed of the leadscrew
corresponding to a table speed of 500 mm/min can be determined fromSlide48
Closed Loop Motion Control System
Uses feedback measurements to confirm that the final
position of the worktable is the location specified in the
programSlide49
Optical Encoder
Device for measuring rotational position and speed
Common feedback sensor for closed-loop NC controlSlide50
Example: Closed Loop
An NC worktable operates by closed-loop positioning. The system consists of a servomotor, leadscrew
, and optical encoder. The leadscrew has a pitch = 6.0 mm and is coupled to the motor shaft with a gear ratio of 5:1 (5 turns of the drive motor for each turn of the leadscrcw). The optical encoder generates 48 pulses/rev of its output shaft. The encoder output shaft is coupled to the
leadscrew
with a 4:1 reduction (4 turns of the encoder shaft for each turn of the
leadscrew
). The table has been programmed to move a distance of 250 mm at a feed rate = 500 mm/min. Determine (a) how many pulses should be received by the control system to verify that the table has moved exactly 250 mm, (b) the pulse rate of the encoder, and (c) the drive motor speed that correspond to the specified feed rateSlide51
aSlide52
Precision NC positioning
Three measures of precision:
1.
Control resolution - distance separating two adjacent
addressable points in the axis movement
2.
Accuracy - maximum possible error that can occur
between the desired target point and the actual position
taken by the system
3.
Repeatability - defined as
±3σ
of the mechanical error
distribution associated with the axisSlide53
PrecisionSlide54
Example: Control Resolution, Accuraq
, and Repeatability in NCSlide55